Various kinds of combustible materials are contained in a secondary battery. As a result, the secondary battery may be heated or explode due to overcharge of the secondary battery, overcurrent in the secondary battery, or other physical external impact applied to the secondary battery. That is, the safety of the secondary battery is very low. Consequently, safety elements, such as a positive temperature coefficient (PTC) element and a protection circuit module (PCM), to effectively control an abnormal state of the secondary battery, such as overcharge of the secondary battery or overcurrent in the secondary battery, are disposed on a battery cell in a state in which the safety elements are connected to the battery cell.
Meanwhile, a battery pack for laptop computers is configured to have a structure in which several battery cells having relatively low capacity are connected in series or parallel to each other due to technical and equipment-based limits of the secondary battery and low safety and yield of the secondary battery, thereby providing a desired capacity to the battery pack.
In recent years, battery packs for laptop computers configured to have 2 parallel (P)-2 series (S), 2P-3S, and 2P-4S structures have been developed. Such battery packs provide various capacities ranging from approximately 28 Wh to approximately 90 Wh.
That is, it is difficult to manufacture a high capacity battery cell due to technical limits of the battery cell constituting a battery pack. For this reason, several low capacity battery cells (for example, 2.5 amperes) are connected to provide a desired capacity to the battery pack.
For example, FIG. 1 is an exploded view typically showing a conventional battery pack having a 2P-2S structure.
Referring to FIG. 1, a conventional battery pack 100 is configured so that four battery cells 30 are electrically connected to each other in a 2P-2S structure. A protection circuit module 40 and a flexible protection circuit board (FPCB) 42 are connected to the upper ends of the battery cells to control the operation of the battery pack.
Also, double-sided adhesive tapes 34 are provided between the battery cells 30 and between the battery cells 30 and a lower case 10. The battery cells 30, the protection circuit module 40, the flexible protection circuit board 42 are sealed by an upper case 20 and the lower case 10.
In addition, the protection circuit module 40 includes an additional element (not shown) to adjust voltage unbalance between the battery cells due to series electrical connection between the battery cells.
In the above battery pack structure, however, cell voltage unbalance between the battery cells connected in parallel to each other must be considered during the operation of the battery pack since the battery cells are connected in parallel to each other, which complicates the design of the protection circuit. Also, costs related to addition of parts constituting the circuit are increased, and therefore, costs necessary to manufacture the battery pack are increased. Furthermore, a battery pack defect rate is increased due to cell voltage unbalance exceeding a malfunction and allowance range of the protection circuit.
In the battery pack manufacturing technology as described above, low capacity battery cells are connected to each other to manufacture the battery pack. Consequently, wiring for series and parallel connection between the battery cells is complicated due to the increase in number of the battery cells, which complicates assembly of the battery pack with the result that productivity is lowered, and a defect rate based on poor assembly of the battery pack during production is increased.
Therefore, there is a high necessity for a battery pack having a specific structure that can provide high capacity without parallel connection between battery cells.
Meanwhile, a battery pack mounted in a laptop computer requires high power and capacity. To this end, a conventional cylindrical battery pack including a plurality of cylindrical battery cells has generally been used. In recent years, however, the size of a laptop computer has been reduced, and therefore, there is a high necessity for a slim type battery pack.
Therefore, there is a high necessity for a technology that is capable of using high capacity pouch-shaped battery cells to manufacture a slim type battery pack, thereby increasing overall current capacity of the battery pack.